The present invention is drawn to new classes of advanced neutron absorbing structural materials for use in spent nuclear fuel applications requiring structural strength, weldability, and long term corrosion resistance.
Throughout the world, reliance on nuclear power generation has been increased in recent years due to a corresponding electric power demand increase. Thus, the amount of nuclear fuel elements handled before and after use has also been increased. The reprocessing of Department of Energy (DOE) spent nuclear fuel was discontinued and DOE spent fuel inventories now require long term storage and disposal. Such demand has created a need for materials to be developed which have sufficient thermal neutron absorption ability and sufficient corrosion resistance for uses in the areas of transportation or storage of nuclear fuels.
Structural materials are needed that will absorb thermal neutrons for criticality control in spent nuclear fuel storage systems. These materials preferably should also exhibit excellent corrosion resistance and good weldability. These materials are used to prevent thermal neutrons from initiating an unwanted nuclear chain reaction. Furthermore, for preventing such container materials from undergoing damage by corrosion, it is generally required that the base metals and weld zones of the materials have excellent corrosion resistance.
Austenitic stainless steel, especially stainless steels having a high chromium-nickel composition, have been used as materials for structural members in nuclear reactors because these stainless steel have good corrosion resistance and acceptable mechanical properties. Borated stainless steels have been developed as structural materials for such applications, because boron (B) has a large absorption cross section for thermal neutrons. These stainless steels can be fabricated to be high strength structural neutron absorbing alloys. However, borated stainless steels have limited usefulness because of known metallurgical problems. For example, such materials can be difficult to weld in structural applications.
Hot workability, cold workability, toughness, and weldability are considerations that should be considered when formulating a metal alloy. Gadolinium is known to have a large neutron absorption cross section. In fact, gadolinium has a neutron absorption ability that is more than four times as great as that of boron. Gadolinium (Gd), is a silver-white, malleable, ductile, and lustrous rare-earth metal that is found in gadolinite, monazite, and bastnasite ores. Generally, it is paramagnetic at room temperature but becomes strongly ferromagnetic when cooled. At room temperature, gadolinium crystallizes in the hexagonal, close-packed alpha form. Upon heating to 1235xc2x0 C., alpha gadolinium transforms into the beta form, which has a body-centered cubic structure.
Because gadolinium has the highest thermal neutron capture cross-section of any known element (about 49,000 barns), attempts have been made to incorporate gadolinium into alloy products for neutron absorbing structural material. For example, in U.S. Pat. No. 3,362,813, a stainless steel alloy containing a minimum of 5% ferrite is disclosed. However, when making a steel product according to the formulas disclosed therein, particularly when using modem steel making techniques, high corrosion resistance is very difficult or impossible to achieve. Additionally, in U.S. Pat. No. 3,615,369, an austenitic stainless steel alloy is disclosed. However, some of the ranges of components disclosed with respect to that composition are not within the useful ranges disclosed herein. Thus, nothing in the prior art appears to teach the compositions disclosed herein, particularly with respect to the low amount of ferrite in the austenitic stainless steel alloys, and with respect to the nickel-based alloys.
The present invention is drawn to a wrought austenitic stainless steel alloy comprising: a) gadolinium at from about 0.1% to 4% by weight; b) chromium at from about 13% to 18.5% by weight; c) molybdenum at from about 1.5% to 4% by weight; d) manganese at from about 1% to 3% by weight; e) nickel at from about 10% to 23% by weight; f) residual amounts of phosphorus, sulfur, silicon, carbon, and nitrogen; g) a balance of material substantially comprising iron, wherein the ferrite content is less than 5% by weight, and wherein the hot forming range is within from about 800xc2x0 C. to 950xc2x0 C. In this temperature range, the alloy is useful for making plate, sheet, strip, bar, and rolled or extruded shapes.
A spent nuclear fuel storage system is also disclosed which is configured for thermal neutron absorption and corrosion resistance. This system comprises a poisoned member being substantially comprised of a cast austenitic stainless steel alloy. The alloy formulation comprises: a) gadolinium at from about 0.1% to 4% by weight; b) chromium at from about 13% to 25% by weight; c) molybdenum at from about 1.5% to 4% by weight; d) manganese at from about 1% to 3% by weight; e) nickel at from about 10% to 25% by weight; f) residual amounts of phosphorus, sulfur, silicon, carbon, and nitrogen; and g) a balance of material substantially comprising iron, and wherein the ferrite content is from about 2% to 25% by weight. Additionally, wrought and cast nickel-based alloys are also disclosed, each comprising: a) gadolinium at from about 0.1% to 10% by weight; b) chromium at from about 13% to 24% by weight; c) molybdenum at from about 1.5% to 16% by weight; d) iron at from about 0.01% to 6% by weight; e) residual amounts of manganese, phosphorus, sulfur, silicon, carbon, and nitrogen; and f) a balance of material substantially comprising nickel wherein the nickel is present at greater than 50% by weight. Furthermore, tungsten may be present in the range from about 0.0% to about 4.0%. In the case of the wrought nickel-based alloys, the alloys should have a hot forming range from about 800xc2x0 C. to 1200xc2x0 C. In one embodiment, the iron content can be restricted to from about 0.01% to 3% by weight.